Shedding some light on light

Today it’s time to take a look at what allows us to look at things. In short, we’re going to look at light.

What we commonly think of as “light” is only a small part of the story. Let’s start at the beginning (where else?).

Light is a form of electromagnetic radiation. To explain: Magnetism and electricity are, in a manner of speaking, two sides of the same coin. Electricity can be converted into magnetism, and vice-versa.

That’s why we can make electromagnets and MRI scanners, and it’s why we can recharge our car batteries while driving and make industrial power plants. We refer to all parts of the phenomenon as electromagnetism.

The shortest waves that we know of are the X-rays and gamma rays, which are the most dangerous to us; their wavelengths are on the order of a billionth of a meter or less.

The longest waves are radio waves, which can be a few feet to more than a mile long.

What we think of as light — that is, visible light — falls about in the middle of the EM spectrum (wavelengths about one millionth of a meter).

To one side (invisible to us) is infrared light — which is what’s given off by hot things — and on the other side is ultraviolet (also invisible to us, but visible to many other animals).

It’s not unreasonable to think of X-rays and radio waves, like infrared and ultraviolet, as kinds of light that we can’t see.

One of the curious things about EM radiation is that it is made of photons. Just understanding what a photon is can be tricky. For convenience, we think of them as little particles, “packages” of light.

But a photon is a “particle” that has no mass. It’s a “package” of energy. You’ll recall that EM radiation moves as waves. This means that the “particles” of light are also waves.

It’s one of the really odd things that physics has taught us: Light is both a particle and a wave. It can be hard to wrap your mind around that one, but that’s how it is.

Visible light includes a number of colors, each of which represents a different wavelength. (The shorter the wavelength, the bluer the light; longer is redder.) In our daily experience, the light that we live by is white light, which is a blend of the various wavelengths that are visible to us.

You might remember learning the name “ROY G. BIV" in science class — representing Red, Orange, Yellow, Green, Blue, Indigo, and Violet, the seven colors that make up visible light.

People have known about these components of white light for a long, long time; in the right circumstances, drops of water refract light to produce the colors (rainbows, for example), and Sir Isaac Newton investigated light using glass prisms to break it up into its component colors.

Long ago, when the astronomer Sir William Herschel was investigating light, he, too, used a prism, projecting the color spectrum to investigate the properties of the colors, including their temperatures.

He noticed that the light grew warmer as one moved from blue to red, and that the area past the red — where there was no visible light — was even warmer. He called this “calorific rays”; we know it as infrared light. Curiosity and investigation — that’s how science works.

We were taught in school that we see objects because they either emit or reflect light: The sun, light bulbs, etc., emit light, and everything else that we see reflects light — blue things reflect blue light and absorb all the other colors; green leaves reflect the green light, etc.

Well, it turns out that this is not entirely accurate. All of the white light is absorbed by the green leaf (for example); the atoms in the leaf’s chlorophyll molecules are excited by the energy of the absorbed light, with some the energy being used to power the leaf’s chemical processes and the rest being re-emitted as green photons.

So reflection is not light merely bouncing off a surface; it’s really absorption and re-emission. Those are new green photons (well, new photons that have wavelengths that we see as green) coming from that leaf.

Light (well, all EM radiation) moves really fast compared to us. I mean really, really fast. The fastest that anyone has ever traveled (relative to the Earth) is on the order of 25,000 miles per hour, and that was to escape Earth’s gravity to get to the Moon.

In most cases, light can’t even imagine going that slowly. In a vacuum, light travels about 186,000 miles every second.

Think of it: In the time that it took you to read “every second” just now, a photon traveled 186,000 miles through empty space. Even 25,000 miles an hour is less than one percent of one percent of the speed of light.

Light is the fastest thing that exists; according to the standard model of physics, it is the fastest thing that can exist.

There a lot more that could be said about light, but there’s far too much to cover in one article, or even in one textbook.

For now, let’s be satisfied to say that light is an amazing phenomenon; like so much of science, the more one learns about it, the more interesting it becomes.